Until a few years ago, orthodontic treatment required the use of metal bands and stainless steel brackets on the teeth. In addition to being unsightly, this arrangement led to irritation of the gingiva and tongue, causing discomfort to the orthodontic patient. In addition, decalcification was often caused by the chemical reaction of band cement with tooth enamel, or by the action of microorganisms encouraged by submarginal food entrapment. These disadvantages were largely eliminated by the introduction of adhesives that had rapid set time and sufficient strength to hold the brackets in place on the teeth, eliminating the need for bands.
The development of an adhesive that is practical for securing brackets in place is difficult. The demands on such an adhesive as to physical and mechanical properties, and as to acceptability in the oral cavity, are rigorous.
Teeth are normally subjected to complex mechanical forces of shearing, grinding and impact in normal biting and chewing operations. In orthodontic treatment additional stress is imposed from the force applied by the arch-wires. A satisfactory bracket adhesive must have good resistance to shearing, grinding and impact, high tensile strength and adhesive bond strength, and must be non-brittle and resilient, so that the bracket will not break off under normal stresses in the orthodontic patient's mouth. The adhesive must also be non-toxic, non-irritating and resistant to the oral environment. A desirable adhesive should be capable of stably and retentively bonding both metal and plastic brackets to tooth surfaces for the period of the orthodontic treatment.
Adhesives heretofore used for bonding plastic or metal brackets to tooth enamel have generally been based on mono- or di-methacrylate resins and have been of two types, i.e.:
(1), a two component self-curing system consisting of a liquid and a powder, two pastes, or a paste and a liquid, which require intimate mixing prior to application to effect self-curing, or
(2), a UV-cured system which cures upon exposure to light with a wavelength of 300-400 nm.
The self-curing systems have the disadvantage of limited working time. UV-cured adhesives depend upon costly and not always reliable light sources, and in addition, cannot be used with some types of brackets. In addition, the UV-curable adhesive material often does not cure properly between the confronting surface of the bracket and tooth enamel, since the adhesive layer is shielded from direct radiation.
U.S. patent application, Ser. No. 845,739, filed Oct. 26, 1977, describes an orthodontic bracket adhesive that requires no premixing and no special curing apparatus, and that can be used for bonding both metal and plastic brackets to etched tooth enamel. This adhesive consists of two components. The first component has a viscosity of between 500 cps and 750,000 cps at 23.degree. C. and contains mono-, di-, or poly-methacrylate or acrylate ester monomers, preferably a glycidyl ester such as glycidyl methacrylate or glycidyl acrylate, or methyl methacrylate to assure proper bonding. Soluble thickening agents, fillers and cross-linking monomers may be present. Either an amine-type accelerator or a peroxide catalyst must also be present.
The second component is more viscous, preferably above 1,000,000 cps at 23.degree. C., and contains mono-, di-, or poly-functional methacrylate or acrylate ester monomers plus a peroxide catalyst or an amine-type accelerator depending upon which the first component contains.
The viscosity of the two components is critical for the performance of the invention. The viscosity of the first component is important to assure the proper penetration of the etched tooth surface, the migration of free radicals to provide a complete and uniform cure of the adhesive, and to prevent slippage of the bracket on the tooth surface before the adhesive has cured. The second component's viscosity of above 1,000,000 cps permits the bracket to be maintained in position on the vertical tooth surface during curing of the adhesive without the need for holding it in position.
In addition, the presence of glycidyl methacrylate (GMA) has been regarded as an essential component when the adhesive is to be used with polycarbonate brackets, to provide adequate bond strength. GMA is capable of solubilizing the polycarbonate resin, which enhances the adhesive bond that is developed.
While generally the adhesive compositions described in the copending patent application have been used with great success, there is a need for other, comparable adhesive formulations, that will perform as well or better, but that utilize different monomers, so as to offer the public and the dental profession in particular greater choice.